The objective of this project is to elucidate the cellular mechanisms responsible for transmission of tublo-glomerular feedback signals. The feedback mechanism is a powerful intrarenal control mechanism that serves to regulate glomerular filtration rate. The macula densa cells of the distal tubule appear to detect increases in tubular fluid composition through increases in cytosolic calcium concentration and transmit signals which increase vascular resistance and decrease filtrate formation. In order to elucidate the sequence of events in the transmission of feedback signals, in vivo micropuncture studies in anesthetized rats and rabbits and in vitro studies of the isolated and perfused thick ascending limb-glomerulus obtained from rabbit kidneys will be performed. Using in vivo micropuncture, the interaction of calcium and prostaglandins in the mediation of feedback signals will be examined. In particular we will determine whether increases in cytosolic calcium result in the formation of a prostaglandin with vasoconstrictor properties. The isolated perfused tubule studies will use fluorescence microscopy to evaluate cytosolic calcium concentration in cells of the glomerulus and in cells of the macula densa. Changes in calcium dependent Quin 2 or Fura 2 fluorescence intensity will be measured with a photometer system. We will evaluate whether or not calcium dependent glomerular fluorescence changes occurring in response to alterations in luminal osmolality can be used as an in vitro index of tubulo-glomerular feedback responses. In addition, we will directly test for the existence of a macula densa cytosolic calcium system by measuring Fura 2 fluorescence in the macula densa during alterations in luminal osmolality. Finally we will evaluate the role of membrane electrical potential in the transmission of signals through the extraglomerular and intraglomerular mesangial cells and smooth muscle by measuring relative changes in fluorescence intensity emitted from membrane potential sensitive dyes. These studies should provide new and important information on the transmission of feedback signals, as well as basic information on communication between dissimilar cells.